[CMake] Installing Python Modules with CMake

[Michael Wild]

On 01/03/2011 06:23 PM, Todd Gamblin wrote:
> On Jan 3, 2011, at 1:34 AM, Michael Wild wrote:
> 
>> On 01/03/2011 09:11 AM, Todd Gamblin wrote:
>>> Is there some standard way in CMake to get a project-local install destination for python libraries, e.g.:
>>>
>>> 	${CMAKE_INSTALL_PREFIX}/lib/python2.6/site-packages
>>>
>>> Is this something find modules typically provide?  I can construct this name easily enough myself, but it involves querying the python executable for its version then regexing that string, and I feel like FindPythonLibs should do stuff like that for me.  It would be nice to have something called, say, "CMAKE_PYTHON_INSTALL_PREFIX" set to this path in advance, so that I can just install python modules there.
>>>
>>> Or am I missing something?  I couldn't fund much on installing python files in the docs.
>>>
>>> -Todd
>>>
>>
>> I think you'll have to query Python yourself. Of course, this becomes
>> difficult if you're cross-compiling, in which case you would need to
>> parse the <prefix>/lib/pythonX.Y/config/Makefile file yourself and fetch
>> all the relevant variables (datarootdir, BINDIR, DESTSHARED, LIBDEST,
>> etc.) and then ask the user what prefix and exec_prefix he wants
>> (presumably CMAKE_INSTALL_PREFIX maps to prefix, and usually exec_prefix
>> is the same as prefix, but that's not always the case). Not sure how
>> that works on Windows, though...
> 
> That's kind of difficult because if I have just 'python', I don't have the version yet, so how am I supposed to find the Makefile?
> 
> Off the top of my head, the most portable way to do this would be to find the python-config (which you can get by appending -config to the exe path, even if it's just python and not pythonX.Y) and then:
> 
> 	head -1 `which ${PYTHON_CONFIG}` | sed 's/.*\([0-9].[0-9]\)$/\1/'
> 
> Or the equivalent in CMake using string(REGEXP ...).
> 
> That will get you the version without running anything, and then you can go and find the Makefile.


That only works on *NIX, on Windows there's AFAIK no python-config. So
if you plan to cross-compile for Windows you might run into trouble.
Probably it's best to not try being über-smart, and just leave it to the
user to provide the information.


> In the non cross-compiled case, though, it would be nice to just have a module that gives you all the information from python-config.

Better run

${PYTHON_EXECUTABLE} -c "import sys; sys.stdout.write(sys.prefix+'\n')"

and similar, which also works on WIN32.

> 
> -Todd
> 
> 
>> I wouldn't rely too much on FindPythonLibs. The whole FindPython* is a
>> sorry mess (IMHO) and needs some serious overhaul, but that is some
>> major undertaking an apparently the suffering wasn't big enough so far
>> for somebody to take action...
> 
> I noticed you guys had a long discussion about finding Python 3.  Did you come to any conclusions on this?  It would be nice to have a python detection Find module that works if you *just* give it your python executable.  I don't mind writing something like this up if no one else is up for it, but if I'm going to write it I'd like to make something that people will use.
> 
> Thoughts?
> 

AFAIK, the issue hasn't been resolved. One of the big problems IMHO is,
that FindPythonInterp and FindPythonLibs can report different python
installations. Also, what should happen if the user manually changes the
cache entry for the interpreter, but doesn't update the libraries? Also,
some users of the module will just want the "most recent version" of
Python, while others will specifically want the most recent 2.x version
because their code isn't ready for python3. If
you can come up with something that works well on all important
platforms and is reasonably backwards compatible, you probably would
make a lot of people happy ;-)

Michael